Field of the Invention
The present invention concerns methods and apparatuses for magnetic resonance imaging, and in particular to a method and a magnetic resonance apparatus for acquiring calibration data or reference data for use in reconstructing a magnetic resonance image from raw data acquired from an examination subject.
Description of the Prior Art
MR imaging is a widely used imaging modality for medical diagnosis as well as for material inspection.
In a magnetic resonance apparatus, the examination object (a patient, in the case of medical magnetic resonance imaging) is exposed to a strong and constant basic magnetic field (called the B0 field), by the operation of a basic field magnet of an MR scanner, in which the examination object is situated. The MR scanner also has a gradient coil arrangement that is operated in order to activate gradient fields that spatially encode the magnetic resonance signals. The magnetic resonance signals are produced by the radiation of radio-frequency (RF) pulses from an RF radiator, such as one or more antennas, in the MR scanner. These RF pulses excite nuclear spins in the examination object, and are therefore often called excitation pulses. The excitation of the nuclear spins at an appropriate frequency causes the nuclear spins to deviate, by an amount called the flip angle, from the alignment of the nuclear spins that was produced by the basic magnetic field. As the nuclear spins relax, while returning to alignment in the basic magnetic field, they emit MR signals (which are also RF signals), which are received by suitable RF reception antennas in the MR scanner, which may be the same or different from the RF radiator used to emit the excitation pulse.
The acquired MR signals are digitized and entered into an electronic memory, organized as k-space, as k-space data. The k-space data are also referred to as raw MR data. The k-space memory has a number of individual locations that are available for entering the digitized signal thereat (data entry points), and the path of data entry points along which the digitized data are entered is called the k-space trajectory. The acquired data can be entered into k-space linearly (line-by-line of data entry points) or radially, along a straight or curved path proceeding from the center of k-space toward the periphery of k-space.
Many reconstruction algorithms are known for operating on the k-space data to convert the k-space data into image data representing an image of the volume of the examination object from which the raw MR data were acquired. This reconstruction algorithm is executed in an image reconstruction computer, resulting in an image data file from the computer that can be shown at the display screen of a monitor, or archived for storage.
After the nuclear spins have been flipped by the RF excitation pulse, the resulting MR signal exhibits an exponential decay in strength as the excited nuclear spins relax. This decaying signal is referred to as an echo signal, or simply as an echo. A commonly used data acquisition sequence of appropriately timed RF excitations and gradient pulse activations (switchings) is the echo planar imaging (EPI) sequence. In an EPI sequence, instead of measuring only one echo after each excitation pulse, multiple echoes are detected by multiple activations of the readout gradient after a single excitation pulse. In an EPI sequence, therefore, the total echo time of the decaying MR signal that follows the excitation of the nuclear spins is divided into a number of individual echo times, corresponding to the number of activated readout gradients.
The activation of sufficient magnetic resonance data (raw data) from an examination subject in order to be able to reconstruct an image of sufficient quality for most diagnostic purposes can be time consuming. Many patients find the amount of time that is necessary for the patient to be in the magnetic resonance data acquisition scanner, while the raw data are being acquired, to be an unpleasant experience because most magnetic resonance data acquisition procedures have, to varying degrees, a certain amount of noise, heat generation and claustrophobia associated therewith. It is also usually necessary for the patient to remain as motionless as possible for the duration of the data acquisition.
Additionally, from the point of view of the hospital or clinic that operates the magnetic resonance imaging apparatus, it is desirable to shorten the amount of time associated with each individual examination, so that patient throughput can be increased in order to facilitate recovery of the high cost of such a magnetic resonance apparatus.
Therefore, there is a continuing effort to decrease the amount of time that is necessary to conduct at least certain types of magnetic resonance imaging examinations.
Among the various techniques for shortening the duration of magnetic resonance data acquisition procedures are the GRAPPA (GeneRalized Autocalibration Partial Parallel Acquisition) and the simultaneous multislice (SMS) technique. These techniques, among others, can be generically called accelerated echo planar imaging, and require calibration data or reference data to be acquired prior to the imaging scan (i.e., the scan in which the actual raw data are acquired) in order to then be able to reconstruct an image from the acquired raw data. In the GRAPPA sequence, raw data are acquire simultaneously with a number of individual local coils. None of the individual local coils acquires enough raw data in order to reconstruct a complete image of the examination region therefrom, and so the respective individual sets of raw data acquired with the local coils must be combined in the reconstruction algorithm. In order to do so, it is necessary to acquire calibration data that identifies the sensitivity or the geometry or the location of the individual local coils.
In the SMS technique, raw data are acquired simultaneously from multiple slices of an examination subject, the multiple slices collectively forming the entirety of the region of interest of which an image is to be reconstructed. This is also a parallel acquisition technique, wherein the respective sensitivities of the multiple coils that acquire the raw data must be known, and this information therefore must be obtained in a calibration scan conducted prior to the imaging scan.
The reduction in the amount of time that is necessary to acquire a complete set of raw data, in order to then be able to reconstruct an image that does not exhibit aliasing, is quantified as the acceleration factor. For in-plane GRAPPA, fully sampled, low resolution datasets of each slice are acquired. As soon as the acceleration factor exceeds two, the data must be acquired in interleaved fashion in order to provide similar geometric distortion properties as will occur in the subsequent accelerated imaging scan.
An earlier conventional version of this data acquisition technique is shown in FIG. 1, designated “conventional.” As shown in FIG. 1, the first segment (Seg. A) is acquired for the entire stack (composed in this example of Slice 1, Slice 2 and Slice 3). Afterwards the second segment (Seg. B) is acquired for the entire stack, and so on. For SMS imaging, the entire slice stack must be acquired in single-band fashion. The reconstruction kernels for the collapsed slices in the subsequent accelerated imaging scan are then calculated based on this reference or calibration scan.
Both gross patient motion and physiological motion (such as respiration) can severely corrupt this calibration scan, which might lead to artifacts in the reconstruction of the subsequent image data. This is particularly problematic for BOLD (Blood Oxygenation Level Dependent) imaging and diffusion acquisitions with multiple diffusion directions and averages, because every subsequently acquired imaging volume relies on the same separately acquired calibration data. calibration data acquired pre-movement may no longer be accurate after a movement has occurred.
Several ways to address these drawbacks have been proposed. One approach is the FLEET reference scan for GRAPPA calibration with high acceleration factors, as described by Polimeni et al., “Reducing Sensitivity Losses Due to Respiration and Motion in Accelerated Echo Planar Imaging by Reordering the Autocalibration Data Acquisition” Magnetic Resonance in Medicine, Vol. 75 (2016) pp. 665-679. The basis of this reference scan is to acquire all segments within one slice first, before proceeding to the next slice, as schematically illustrated in the FLEET sequence shown in the bottom portion of FIG. 1. This is achieved by utilizing special flip angle combinations.
Another approach extends the FLEET reference scan to multislice imaging, as proposed by Bhat et al., “Motion Insensitivity ACS Acquisition Method for In-Plane and Simultaneous-Multi-Slice Accelerated EPI,” Proc. Intl. Soc. Mag. Reson. Med. 22 (2014), 644. In this extension of the FLEET reference scan, calibration data for slices that are to be acquired simultaneously in the subsequent imaging scan are acquired closer in time, in order to minimize the change of corrupted data acquisition. This approach is schematically illustrated in FIG. 2 for a two-segment acquisition and a stack of four slices wherein slices 1 and 3 are acquired simultaneously later one. The RF excitation pulses for the two segments are respectively designated α1, α2 and the subsequent echo signals (ADC signals) are also shown, with the associated readout and phase encoding gradients.
As can be seen from the bottom timeline in FIG. 2, the imaging readout for each slice takes place immediately after each block excitation.